A microfilter, sometimes referred to as a "DSL microfilter" or an "ADSL microfilter", is a small device that is plugged into a UK (United Kingdom) phone socket. The purpose of a microfilter is to enable 'standard broadband' -- which in the UK is the DSL/ADSL technology -- to work in combination with a landline telephone service; telephone equipment is not built to understand an ADSL signal. Prior to DSL/ADSL, the standard Internet connection technology in the UK was dialup, which when in use, rendered the standard landline telephone service as offline/inactive. Therefore, DSL/ADSL, with its microfilter device, was not only faster than dialup, it had the added advantage of providing two simultaneous network services. A microfilter is easy to setup: it is slotted into a UK phone socket (shown below), and then a RJ45 cable/plug (phone) and a RJ11 cable/plug (router/modem) is inserted into the microfilter.
(Pictured: A standard ADSL microfilter and phone, that costs in the region of £2- £5 respectively)
Microfilters are not an expensive piece of equipment: they cost in the region of £2-£5, and are typically included with a DSL/ADSL router/modem. The vast majority of new microfilter are compatible with ADSL 2+, which is the fastest version of ADSL, and was introduced around 2003-2005. Due to the majority of microfilters being inexpensive, and being in constant use, they do have a tendency to degrade over time. Therefore, if an ADSL connection is disconnecting on a regular basis, or there is static on the line, the first thing that is usually recommended by ISPs is to replace the microfilters; even though the reason is not always due to the microfilters and may be the wiring outside/inside the home. As stated, the cheapest microfilter will typically cost between £1.50-£2.00, but 'professional' microfilters cost in the region of £5-£6 and provide compatibility for PABX systems and DECT cordless phones.
The image shown above is an example of a standard microfilter -- that is most commonly used in the UK -- but there are other types, such as: a micro plug-in adapter (half the size), and a master/phone socket with the microfilter built into it. Are microfilters a requirement if someone is only planning to use the phone line for broadband Internet? BT suggest "you may get problems with your BT Broadband or hear noise on your phone line" if microfilters are not used, and manufacturers of routers (Netgear) warn that without a microfilter the "ADSL line is not terminated correctly" and may produce "reflections back down the line". Therefore, while DSL/ADSL broadband may work without a microfilter, it is generally not recommended. Whether microfilters will need to be used on every phone socket will depend on how the phone wiring is setup in a home; however, the master socket will require a microfilter.